Silicon dioxide thin films are ubiquitous in semiconductor technology and have been since its inception. There is an immense background of experience and technology in processes for depositing SiO.sub.2 thin films, and for their application. The uses have been primarily for dielectric layers for semiconductor passivation, for defining localized impurity regions in semiconductor processing, and for interlevel dielectrics. In optical device technology SiO.sub.2 is a principal waveguide material, and is used in combination with other optical materials, both passive and active. These films have been deposited by evaporation, chemical vapor deposition, and sputtering.
Modifying electrical properties of SiO.sub.2 using various dopants is a technique widely used in semiconductor technology. By implanting SiO.sub.2 layers with n- or p-type dopants, the surface characteristics of silicon beneath the SiO.sub.2 layers can be altered. Relatively small doses of dopants, well within the capability of conventional ion implantation methods, are useful in producing these electrical effects.
Applications in optical device technology have been proposed for doped oxide films to alter e.g. electronic conductivity, ionic mobility, refractive index, but the doping levels to achieve useful results in some of these applications are high, and some of the desired dopant species, e.g. oxides, are unconventional in the context of standard ion implantation methods. Moreover, some optical applications require high compositional uniformity in the doped material, a result not typically obtained using the methods commonly used for semiconductor processing.
Doped SiO.sub.2 films can have altered ionic mobility with a wide range of activation energies. In particular, lower voltage drift can be achieved if ionic mobility is suppressed (i.e. In or P doping). In contrast, V.sub.2 O.sub.5 doped SiO.sub.2 films have a very high ionic mobility making these films suitable as solid electrolytes.
Electronic conductivity can be increased by several orders of magnitude when SiO.sub.2 thin films are lightly doped with In, Al, Ti, etc. with only minor changes of the dielectric constant and the index of refraction. Consequently, the electrical properties of high silica dielectric materials can be tailored to a specific application. A well controlled variation in the index of refraction is also possible and may be used for waveguide applications. Multivalent elements may be electrochromic when doped into silica. They can be used as lightvalves, or for display applications.
Many compositions of interest for optical device applications are not stable in the glass form. Therefore, a popular approach for preparing such films is to sputter the composite film from a target composed of tiles of the individual component materials. The fractional area of the individual targets are adjusted using the respective sputtering yields to result in the film composition desired. Unfortunately, the sputtering yields of different materials shows remarkably different dependence on the deposition parameters (such as rate, pressure, plasma potential, reactive gases, etc.). Therefore it is difficult to optimize deposition conditions to control film properties such as stress, hermeticity, moisture resistance, grain size, etc. as well as reduce particle formation and process cost, and still produce the desired composition.
An approach that is designed to overcome some of these deficiencies is to co-sputter the films from individual targets of component materials using separate sputtering controls for each target. This method is complex and expensive. The physical arrangement of the targets adds further complications due to the variations in distance and angle between the individual targets and the substrate, resulting in shadowing effects when surface mobility is limited and deposition rates vary significantly (as is common with doped silica films).
Theoretically, the best approach is to sputter from a composite target, Which is composed of fine particles of materials having the different compositions. Each material may be vitreous or crystalline and have one or several glassy or crystalline phases. If the particles are sufficiently small, and if the sputtering process is allowed a transition period to a steady state sputtering condition, the composition of the composite target is reproduced in the sputtered film. Due to the rapid formation of the film it remains in the vitreous state.
However, while this sputtering process would appear to be highly attractive for commercial manufacture, sputtering targets with mixed compositions of interest are not available with the requisite compositional uniformity and particle size. This is especially the case with highly doped silica targets. To date, conventional methods for target preparation, typically ceramic or glass forming methods, cannot produce targets with high silica content and sufficiently small particle size to allow replication of the target composition in the sputtered films. Attempts to overcome the particle size limitation by using powder targets have been tried. However, powder sputtering targets are difficult to cool, which makes them suitable only for low rate deposition, and can only be used in apparatus in which the sputtering direction is vertically upward.
In summary, while sputtering from a composite target would appear to be the best process for preparing doped silica films, the lack of suitable targets makes this process solution commercially unattractive.